Rtp - invite

Im making calls between 2 nokia N80 in the same subnet with Asterisk. I
want direct RTP traffic between the terminals but I’ve noticed that before
that happens there is a short RTP flow between the terminals and Asterisk.
Why is that?
I’ve also noticed that there are several SIP INVITE messages during the
RTP flow between terminals. Why does it send more than the initial INVITE?